Fears chemical attack on Sydney commuters was next after alleged 'plane bomb'

After the plan to blow up the plane mid-flight was thwarted at check-in, police are looking at the possibility that the suspects regrouped, sought fresh orders from Islamic State operatives overseas and then plotted to unleash a public transport chemical attack in Sydney.

While much of the focus had been on protecting our skies, there are new concerns that buses, trains, and ferries were part of alleged plan to murder Sydney commuters with deadly hydrogen sulphide.

Police allege brothers Khaled and Mahmoud Khayat were being directed by an Islamic State controller in Syria.

"This is one of the most sophisticated plots that has ever been attempted on Australian soil,” Deputy AFP commissioner Michael Phelan told reporters.

Police are also examining the possibility that the alleged terorrists were planning an attack on congested Sydney locations such as trains and buses. Source: 7 News

Explosives were shipped from Turkey to Sydney in an alleged attempt to bring down an aircraft taking off from Sydney airport. Source: 7 News“They were talking about crowded closed spaces, potentially public transport."

"How to create the device, how to create the chemical reactions, indeed, the amount of chemicals that would be used to create the most amount of damage within a confined space."

Police claim they've recovered ingredients for the toxic compound, and components for a device being built to house it, at several addresses.

Police have two more days to question him.

"We will take our time and when the time's right to make that decision, we will,” Deputy NSW Police commissioner David Hudson added.

Police raided properties around Sydney, finding equipment inteded to cause mass damage. Source: 7 NewsAlso found at the alleged conspirators' homes were components of a meat grinder, allegedly used to conceal the world's most dangerous plastic explosive, Pentaerythritol tetranitrate, or PETN –the same explosive used by infamous shoe bomber, Richard Reid.

Police allege Khaled Khayat arrived at Sydney Airport with his brother Ahmir on July 15 who checked onto an Etihad flight clueless brother Khaled had packed a bomb in his luggage.

Police don’t know why the bag wasn’t checked in but Ahmir continued onto the flight without it, unaware of his brothers' alleged plans to murder him along with hundreds of others.

It's believed one man had no idea his older brother was planning on bringing down an entire aircraft with them both on board. Source: 7 NewsIt’s understood their other brother, Tarek, an Islamic State fighter in Syria, allegedly introduced an IS controller to the Ahmir.

The explosives were then sent via air cargo from Turkey to Sydney, slipping through Customs, followed by ongoing instructions on how to build a bomb and the most effective way to deploy it.

After the alleged conspiracy was uncovered, police did trial runs of their own 'bomb' passing through airport security.

"We had 100 per cent success rate in terms of our mock IED being picked up."